Depending on country, the franking machines currently on the market are either post-payment machines for performing franking operations during a certain period of time, or else pre-payment machines for performing franking operations up to an amount of credit loaded into the machine.
The postal data of the machine is constituted in both cases by the total value of the franking operations performed, and for prepayment machines only, by the amount of usuable credit remaining in the machine. This data must be accounted very reliably and must be capable of being checked so as to avoid fraud.
As franking operations are performed by a franking machine, an up meter gives the total value of the franking performed. And as postal values are printed, a down meter gives the value of the remaining credit. A cumulative meter gives the total value of the amounts of credit successively loaded into the machine, and is associated with the up and down meters for performing arithmetic checks on the postal data per se.
Each time credit is loaded into the machine, the state of the down meter and the state of the cumulative meter are increased by the amount of the credit. Each time a franking operation is performed, the value of the franking operation is added to the up meter and subtracted from the down meter. The up and down meters therefore change each time a franking operation is performed.
The down meter also serves to lock the machine when insufficient credit remains.
Post-payment franking machines are also equipped with these three meters: up meter, down meter, and cumulative meter. Credit is not loaded into the machine, so the down meter also cumulates the successive franking operation amounts, taken as negative values, and the cumulative meter remains at zero.
In electronic systems for postal data accounting in a franking machine, these meters are constituted by a microcomputer and a non-volatile memory. The operations of updating the total value of franking operations performed and/or the value of the credit remaining are performed by the microcomputer each time a franking operation is performed or each time the machine is reloaded. The memory records the resulting new meter values. These successive values are recorded on m bits, where m is compatible with the maximum value the meter is allowed to reach. This memory is commonly referred to as the "meter" memory or simply the "meters" of a franking machine.
For reasons of operating security, the up, down, and cumulative meters may be doubled up. The data contained in corresponding meters must coincide, and non-coincidence between the meters is used to prevent any further franking operation and to lock up the machine.
For the same reasons, the cumulative meter is associated with the up meter and the down meter. The cumulative meter is used for recording the total value of credit ever loaded into the machine. It serves to provide internal arithmetic checking on proper machine operation, since its contents must always be equal to the sum of the contents of the up meter and the down meter.
In electronic machines, the meter-constituting memory(ies) must satisfy two requirements in particular. Firstly the recorded data must be stored even when the machine is switched off or there is a failure in the machine power supply. Secondly the memory must be capable of being subjected to a number of write cycles which is not less than the number of franking cycles which the machine can perform. In addition, account must be taken of the credit reloading cycles since these cause new values to be written in the up meter and the cumulative meter, but the total number of credit reloading cycles remains relatively much less than the number of franking cycles.
Electronically erasable programmable read only memories (EEPROM) satisfy the first requirement. However, the number of write cycles to which they can be subjected is insufficient in normal operation of the memory, even for the commonest average capacity franking machines on the market.
That is why random access memories (RAM) which can be subjected to an indefinite number of write cycles are used. In order to satisfy the first requirement, they are backed up by a battery which provides the energy they require when the machine is switched off or when its power supply fails.
The use of a battery-backed RAM has a limited lifetime because of the battery. It also requires that the state of the battery be continuously monitored and the battery needs changing from time to time.
The use of a battery also requires auxiliary components to be associated therewith. Such auxiliary components reduce machine reliability. In particular, a fault in one of the auxiliary components may short-circuit the battery, thereby losing all the stored data. A mechanical shock or an accidental increase in temperature may cause battery sealing to rupture by causing its sealed envelope to tear, thereby damaging the battery and possibly releasing various toxic substances into the surrounding atmosphere.
The object of the present invention is to avoid these drawbacks.